Oh, I don't know about that. A few years ago a friend of mine gave me a round inch thick round stainless steel plate to try out some armor piercing ammo on. I had loaded up the 165 grain bullets pulled from old .30-06 ammo into .308 cases. I took my FAL out to my field and set the stainless steel plate out about 50 yards and took a shot at it. I thought I had missed it completely, because I had just stuck it into the ground without any sort of support.
I was VERY surprised when I walked up to the plate and saw that I had certainly hit the thing, dead center even, the steel penetrator just barely breaking through the back of the plate. This plate weighs about 20 pounds (rough guestimate), was hit dead center with a .308 that dumped ALL of it's energy into it and it didn't even wiggle when hit.
Now, I tried the same thing with a .50BMG using regular hardball ammo. Now as far as I know, all hardball .50BMG ammo has a mild steel core (except for the specially designed ammo), so it already has a sort of penetrator. Anyway, one shot dead center on that same plate at 70 yards knocked the plate up into the air and back about 10 feet. The bullet put a jagged hole right through the plate like it wasn't even there.
A friend of mine brought over some old cast iron weights from his weight lifting set to use as targets. He wanted to shoot a .50 BAD! One hit completely shattered those 25 pound weights. All my friend could say after every shot was "impressive!".
I do believe there will be jobs that a .50BMG can do that a .308 couldn't even dream about doing.